The present invention relates to an object with radially-varying properties, which is used to prepare graded-index plastic optical fiber in the field of communication or image transmission. More particularly, the present invention relates to a method of preparing an object with radially-varying properties and an apparatus for preparing the same. The object with radially-varying properties can be prepared with polymers or ceramics. In this invention, the properties mean an optical property such as refractive index, tensile strength, color, heat expansion coefficient, relative concentration of components, effect of catalyst, etc.
An object with radially-varying properties can be used in the field of communication or image transmission or for other purposes. In particular, an object with radially-varying properties has been used as graded-index plastic optical fiber for telecommunication.
The conventional optical fibers for communication systems are classified into single-mode glass optical fibers and multi-mode glass optical fibers. The single-mode glass optical fibers have been widely used as long-distance and high-speed communication media. However, because the single-mode glass optical fibers have small core diameters, typically 5 to 10 microns, extreme accuracy is required in the alignment of the fibers for interconnection with other components of the optical communication system, thereby increasing the costs of the whole system. In contrast to single-mode glass fibers, multi-mode glass fibers, which can have diameters larger than single-mode glass fibers, have been used primarily for short distance transmission such as local area networks (LANs). However, even their moderate cost for interconnections has limited their application. Consequently, metallic cables such as twisted pair or coaxial cable are still used extensively in short range applications, namely up to 200 meters. However, these metallic cables cannot meet the anticipated future bandwidth requirement of several hundred MHz (for example, the asynchronous transfer mode[ATM] standard of 625 megabits per second). There has been considerable interest in developing plastic optical fiber (POF) in the short range communication applications, such as LANs. POF can have core diameters of about 0.5 to 1.0 mm, which makes it possible to adopt injection-molded polymer connectors, drastically reducing the cost associated with interconnecting the POF to the other components of a system. These plastic optical fibers can have a step-index (SI) structure or gradient-index (GI) structure. Unfortunately, step-index plastic optical fiber (SI-POF) suffers high modal dispersion and therefore cannot meet the bandwidth requirements. However, gradient-index plastic optical fibers (GRIN-POF), having low modal dispersion, have high potential to be a high bandwidth, cost effective media for use in short range communication applications.
An interfacial gel polymerization process for preparing GRIN-POF was introduced by professor Koike in 1988 (Koike, Y. et al., Applied Optics, vol. 27, 486(1988)), and thereafter many patent applications were filed: U.S. Pat. No. 5,253,323 to Nippon Petrochemicals Co.; U.S. Pat. No. 5,382,448 to Nippon Petrochemicals Co.; U.S. Pat. No. 5,593,621 to Yasuhiro Koike and Ryo Nihei; International Patent PCT WO 92/03750 G02B6/00 to Nippon Petrochemical Co.; International Patent PCT WO 92/03751 G02B6/00; Japan Kokai Tokyo Koho JP 03-78706 G02B6/00 to Mitsubishi Rayon; Japan Kokai Tokyo Koho JP 04-86603 G02B6/00 to Toray Ind., etc. These processes may be divided into two broad types:
1. Batch processes in which a preform is made with a gradient index and subsequently drawn into a fiber. The preform is made of a polymer(s) plus a low molecular weight additive.
2. Fiber extrusion processes followed by radial extraction of low molecular weight components, and/or radial infusion of molecular weight components, and subsequent polymerization of residual monomer.
The first type of process was successfully implemented in producing fiber with the measured bandwidth of 2.5 Gbits/second. The second type of process has had similar success in achieving a high bandwidth.
In addition to the above-mentioned patents and patent applications, U.S. patent application Ser. No. 89/929,161 (PCT/US97/16172) now U.S. Pat. No. 6,267,915 on a method of preparing GI optical fiber was filed by Park and Walker. The process by Park et al. is achieved by applying a polymeric material having an axial variation of a material property to a rotating cone, which converts the axial variation to the radial variation. Park et al. also disclose an apparatus for producing a cylindrical form with at least one radially-varying material property comprising mechanical means for transforming an axial variation of a material property into a radial properly of the material property.
With respect to using GRIN-POF in LANs and other related applications, the objective is to minimize modal dispersion. The required radial refractive index profile for minimal modal dispersion has been studied extensively. The model (Halley,P. [1987] Fiber Optic Systems, J. Wiley and Sons; Olshansky,R., D. B. Keck [976] Appl.Opt.15(2): 483-491) of a GRIN fiber normally considered is that of a xe2x80x9cpower lawxe2x80x9d index variation:                               n          ⁡                      (            r            )                          =                                            n              1                        ⁡                          [                              1                -                                  2                  ⁢                                                            Δ                      ⁡                                              (                                                  r                          a                                                )                                                              g                                                              ]                                            1            2                                                        for          ⁢                      xe2x80x83                    ⁢          r                ≤        a                                n        2                                      for          ⁢                      xe2x80x83                    ⁢          r                 greater than         a            
where r is the radial distance from the fiber axis, a is the radius of the fiber, n1 and n2 are the refractive indices at r=0 and r=a, respectively, where n1 xe2x96xa1 n2. The parameter g determines the index profile as a function of radius and 2xe2x96xa1=(n12xe2x88x92n22)/n12. In the particular case where g=2, the power law is called the xe2x80x9cparabolic lawxe2x80x9d. When the value of g approaches to 2, an optimum refractive index profile for maximum bandwidth can be obtained. It can be shown that if a light signal in the form of a delta function is launched into a GRIN fiber, the maximum bandwidth, B is given by:   B  =                    c                  0.088          ⁢                      xe2x80x83                    ⁢                      Ln            1                              ·              1                  Δ          2                      ⁢          xe2x80x83        ⁢          (              bits        ⁢                  /                ⁢        second            )      
where L is the length of the fiber, and c is the speed of light.
In theory, the bandwidth of GRIN-POF is extremely sensitive to the value of g near the optimum value. Therefore, in preparing GRIN-POF, how large a bandwidth GRIN-POF has depends on the ability of a process to control the value of g. In conventional processes of preparing GRIN-POF, except the process by Park et al., the refractive index profile in the radial direction is determined by the diffusion of a lower molecular material or the relative reactivity of two materials. Thus the conventional processes do not have the ability to control the value of g or the radial profile of the refractive index. The process by Park et al. above-mentioned claims to have the ability to control the value of g by mechanical mixing of two or more polymers using a particular extrusion mold die. However, the process has disadvantages in that it is difficult to produce optical fiber with a low attenuation due to the complicated structure of the extrusion die and contaminants resulting from the thermal decomposition of polymers from coextrusion process.
Therefore, the present inventors have developed a method of preparing a plastic optical fiber and an apparatus for preparing the same. The process of this invention has the ability to control the refractive index profile. Unlike the process of Park et al., the new process is not an extrusion process and the apparatus is not complicated. The present process can provide a method of preparing a plastic optical fiber with a low intensity loss of a light signal as in the process by Koike.
A feature of the present invention is the provision of an object with radially-varying properties, which is used to prepare graded-index plastic optical fiber in the field of communication or image transmission.
Another feature of the present invention is the provision of a method of preparing an object with radially-varying properties, which is used to prepare graded-index plastic optical fiber in the field of communication or image transmission.
A further feature of the present invention is the provision of a method of preparing an object with radially-varying properties, which has the ability to control the value of g or radial index of refraction.
A further feature of the present invention is the provision of an apparatus for preparing an object with radially-varying properties, which has a simple structure.
A further feature of the present invention is the provision of a method of preparing a plastic optical fiber with a low intensity loss of a light signal.
Other features and advantages of this invention will be apparent from the ensuing disclosure and appended claims.
The method of preparing an object with radially-varying properties in accordance with the present invention, which is used to prepare graded-index plastic optical fiber in the field of communication or image transmission, comprises providing a reaction apparatus comprising an outer container, an inner container installed in the outer container, a rotating rod installed at a position in the inner container, and a sealing member for sealing the outer and inner containers at the bottoms thereof; filling the inner container with an inner material and a space between the inner container and the outer container with an outer material wherein the outer material has different properties from the inner material; removing the inner container; and rotating the rotating rod for laminar mixing of the two materials. The apparatus for preparing an object with radially-varying properties in accordance with the present invention comprises an outer container with a certain cross-section; an inner container with a certain cross-section, installed in the outer container; a rotating rod installed at a position in the inner container, and a sealing member for sealing the outer and inner containers at the bottoms thereof.